Some of the earliest Scene versions of Soundtracker (e.g. Jungle Command / Defjam / DOC Soundtracker II - VI) released in 1988 contain some intriguing commands that take advantage of Paula's ability to use the output from one channel to modulate the volume and/or frequency of a second channel:

3 - Modulate volume of next higher voice.
4 - Modulate period of next higher voice.
5 - Modulate period + volume of next higher voice.
6 - Modulate volume of next higher voice and portamento up.
7 - Modulate period of next higher voice and portamento up
8 - Modulate period + volume of next higher voice + portamento up.
9 - Modulate volume of next higher voice and portamento down.
A - Modulate period of next higher voice and portamento down.
B - Modulate period + vol of next higher voice + portamento down.

Does anyone have any songs that use this modulation functionality?

I have never seen one in the wild, not even a demo on the original Soundtracker II-VI disks(*). This is perhaps not surprising given that (a) it's difficult to control the effect when one sample is used to modulate another and (b) there were only a few short months before these old Soundtrackers were superseded by much more user-friendly versions that evolved towards the “modern” command set. Nevertheless, it would be fascinating to hear an aesthetically pleasing example. Paula's modulation hardware allows some pretty sophisticated sound synthesis if multiple channels are chained together - but software that uses it seems to be extremely rare.

For most purposes the Protracker tremolo and vibrato commands (with preset waveforms) no doubt suffice and have the advantage that no channels are lost (channel that controls the hardware modulation is always silent). However, I'm sure some interesting effects could be achieved with arbitrary modulation waveforms and several modulating channels chained together, and would have better quality than could be practically be achieved in real time with software. (For example, the Protracker replay would typically only update the effect with a resolution of ~50Hz depending on the CIA speed setting.)

It does seems strange that someone went to the trouble of building these features into the early Soundtrackers but never created any songs that use them. There's a very interesting discussion of Soundtracker evolution here by a guy who has written a Tracker replay library in Javascript: http://weaselaudiolib.sourceforge.net/ He hasn't found any historical examples either.

Maybe I'll have a go myself! I might write my own replay utililty too because Soundtrackers II-VI are a real pain to use (break if run on anything other than kickstart 1.2 with cycle-exact A500 timing) and I don't think any other replayers exist that support the legacy commands.

Using these features is accepting losing one or several audio channels, and we already don't have enough of them.
So if a musician has the choice between 4 and software effects vs 2 or 3 and hardware effects, what do you think he'll most likely choose ?

Using these features is accepting losing one or several audio channels, and we already don't have enough of them.
So if a musician has the choice between 4 and software effects vs 2 or 3 and hardware effects, what do you think he'll most likely choose ?

That wildly depends on the use. Losing two channels could still be acceptable.

You could be using the Amiga as a stereo monosynth and losing two channels wold be fine. I for one would love to try that.

As an actual musician, not one trying to make music for a demo or a game, this functionality sounds very tempting. That's why I also questioned its use within the context of a tracker. It would be better suited for something like Aegis Sonix. The Amiga doesn't have enough synth software anyway.

I might write my own replay utililty too because Soundtrackers II-VI are a real pain to use (break if run on anything other than kickstart 1.2 with cycle-exact A500 timing) and I don't think any other replayers exist that support the legacy commands.

It should be fairly simple to patch them to work on another specific Kickstart version (e.g. 1.3), if that could help at all?